Conventionally, as a method of growing a compound semiconductor bulk single crystal are known the vertical Bridgman (VB) method in which a growth vessel (crucible) filled with a melt is gradually moved to grow a crystal in an upward or downward direction in relation to a heating furnace having a temperature gradient or a vertical gradient freezing (VGF) method in which a vertical temperature gradient is provided in a melt and the melt is gradually cooled to grow a crystal.
In the VB method and VGF method, a crucible is typically used in which the bottom of the crucible is inclined down toward the center of the bottom. A bulk single crystal is obtained by locating a seed crystal having the same composition as the crystal to be grown and having a diameter considerably smaller than the crystal to be grown, on the center of the bottom of the crucible, to solidify a melt; or by naturally generating a nucleus at the center of the bottom of the crucible without using the seed crystal to solidify the melt.
As a growth method of a compound semiconductor bulk crystal, a liquid encapsulated Czochralski (LEC) method is known in which a melt is covered by encapsulating material such as B.sub.2 O.sub.3 in the crucible and the seed crystal is brought into contact with the surface of the melt to grow a crystal by gradually pulling up the seed crystal. This method is widely used. The seed crystal used in this method also has the same material as the crystal to be grown and a considerably smaller diameter than it, like the above-mentioned VB method and VGF method.
However, since in the II-VI group compound semiconductor such as CdTe, thermal conductivity is low and shearing stress is small, there is a problem in that a single crystal of good quality cannot be obtained with a high yield in the above-mentioned conventional VB method, VGF method and LEC method. Further, there is also another problem in the conventional VB method and VGF method. Nucleation of polycrystal growth is generated during the growth of crystal shoulder portion corresponding from the center of the bottom of the crucible to a straight portion of the crucible. In addition, since the shoulder portion must be formed in the conventional VB method, VGF method and LEC method, there is another problem in that the straight portion is short so that the number of wafers cut out from the crystal decreases.
In the VB method, VGF method and LEC method, a method is proposed in which a crystal having shorter shoulder portion is grown by using a seed crystal having a diameter greater than that of the conventional methods. However, since the shoulder portion cannot be completely removed, the portion is not yet solved, i.e., nucleation of polycrystal growth is generated during the growth of shoulder, as described above.
Further, because the seed crystal is the same material as the crystal to be grown, there is another problem in that it is very difficult to control the amount of the seed crystal molten by the melt within a predetermined level, wherein a portion of the seed crystal in contact with the melt is molten upon seeding.
The seed crystal which is the same material as the crystal to be grown is typically used but there is a case that the seed crystal which is a different material is used in a special example. For instance, when there is no suitable GaP seed crystal to grow a GaP single crystal, the GaP single crystal is first grown by the LEC method using an InAs single crystal as the seed crystal and then a GaP seed crystal is cut out from the obtained crystal ingot to use in the subsequent manufacture of a GaP single crystal. In this method, however, there is a problem in that a single crystal to be obtained (GaP in the above example) cannot be directly obtained from the seed crystal having the different material (InAs in the above example) and the material of the seed crystal which is the different composition dissolves in the material melt so that a crystal having a different composition is grown.
In the liquid phase epitaxy (LPE) method, there is an example in which a CdTe single crystal film is grown on a sapphire substrate (aluminum oxide: Al.sub.2 O.sub.3). However, a thin film of CdTe crystal is only obtained and a bulk crystal cannot be obtained.
Alternatively, there is a method in which a bulk crystal is grown on the sapphire substrate by a vapor phase epitaxy method. In the method, however, there is a problem in that the growth rate is very slow and a large size of crystal cannot be grown.
As described above, in the conventional VB method, VGF method and LEC method, or a method using a large diameter of seed crystal in these methods, a bulk single crystal of good quality cannot be obtained with a high yield.
The present invention is accomplished in the light of above circumstances. The object of this invention is to provide a method of growing a bulk single crystal of good quality with a high yield.